Since its advantages of simple scheme, low cost, being without high frequency transformer and EMC, LED linear constant current driver circuit is widely used.
The currently used LED linear constant current driver circuit is shown in FIG. 1, wherein the input DC source is connected to the input end of a rectifier bridge 101 and the positive output end of the rectifier bridge 101 is connected to the input end of a LED string 102, the negative output end of the rectifier bridge 101 is connected to the ground. A constant current module 103 is constituted by an amplifier 104 and a N-mosfet 105. The output end of the LED string 102 is connected with the drain of the N-mosfet 105, the gate of the N-mosfet 105 is connected with the output end of the operational amplifier 104 and the source of the N-mosfet 105 is connected with one end of the current sampling resistor 106 and the negative input end of the operational amplifier 104 respectively, another end of the current sampling resistor 106 being connected with ground and the positive input end of the operational amplifier 104 being connected with the reference voltage Vref. FIG. 2 is a simplified timing view of the circuit.
Assuming that the rectified voltage is Vin and the voltage at two sides of the LED string 102 is Vled, when Vin is higher than Vled, there is current passing through the LED string 102 and into the constant current module 103 and forming a voltage signal on the current sampling resistor 106; the operational amplifier 104 inputting signal to control the gate of the N-mosfet 105 by comparing signals at positive and negative input end allows the voltage between the source and drain of the N-mosfet 105 to be dynamically adjusted and the voltage signal at the source of the N-mosfet 105 to be equal to the reference voltage Vref, thus achieving the purpose of the current passing through the LED string 102 constantly.
As such, assuming that the resistance of the current sampling resistor is Rcs and the voltage of the N-mosfet 105 is Vds, Vds=Vin−Vled and the power consumption of the N-mosfet 105 is
      P    =                            V          ds          *                ⁢                              V            ref                                R            cs                              =                        (                                    V              in                        -                          V              led                                )                *                              V            ref                                R            cs                                ;assuming that Vds=Vled, i.e. The voltage difference between the input voltage Vin and the voltage at two ends of the LED string 102 is
            1      2        *          V      in        ,the efficiency of entire driver circuit is 50%. It is shown that the larger the voltage difference between the input voltage Vin and the voltage at two ends of the LED string 102, the larger the power consumption of the N-mosfet 105, and thus the lower the efficiency of entire driver circuit.
If the number of LED lamp in the LED string 102 is increased, the voltage difference between the input voltage Vin and the voltage at two ends of the LED string 102 will decreased, however, increasing the number of LED lamp in the LED string 102 will cause the increasing cost, therefore it is a good method for improving efficiency.
Thus, the problem in prior art is that how to improve the efficient of the whole linear constant current LED driver circuit upon the larger voltage difference between the input voltage Vin and the voltage at two ends of the LED string 102.